Isocyanates, such as toluene diisocyanate ("TDI") and their derivatives, such as thioisocyanates, are commercially important chemicals that are useful in the preparation of urethane foams, urethane elastomers, coatings, insecticides, herbicides, and the like. Isocyanates can be conveniently grouped into the broad categories of aromatic isocyanates and aliphatic isocyanates. Aromatic isocyanates, such as TDI, contain an aromatic group, such as a phenyl ring, in the molecular structure of the compound. Aliphatic isocyanates, on the other hand, include one or more aliphatic groups, such as butyl or cyclohexyl groups. Examples of this type of isocyanate include isophorone diisocyanate (IPDI), 1,6-diisocyanatohexane (HDI) and saturated 4,4'-diphenyl methane diisocyanate (H.sub.12 MDI).
Isocyanates are produced by a variety of commercially established procedures, such as phosgenation of primary amines, reaction of primary amines with carbon dioxide and acyl halide, and reaction of primary amines with carbon dioxide and hexamethyldisilazane. Unfortunately, during the manufacture of isocyanates using these procedures, an appreciable amount of monomeric isocyanate polymerizes to form a polymeric residue as a by-product. Thus, the total reaction product is a combination of desirable monomeric isocyanate molecules and undesirable polymerized isocyanate residue by-product. The polymerized isocyanate residue in the by-product is considered an impurity and reduces the overall yield of the synthesis reaction. In addition, since the by-product typically contains relatively low viscosity isocyanate monomer(s) that is difficult to separate from the polymerized isocyanate residue, the by-product itself is typically a viscous, sticky paste that interferes with processing and bulk transfer equipment. Moreover, since heretofore the isocyanate monomer(s) could not heretofore be easily separated from the by-product, an additional yield loss typically occurs due to these unrecovered monomers in the by-product.
There is a plethora of equipment available in the marketplace for effecting separations of compounds within mixtures, such as evaporators, extractors, and heated mixers. Heretofore, although effective separations have been carried out using aromatic isocyanate residues, the prior art methodology is generally not useful for making separations with respect to aliphatic isocyanate residues.
Illustrative of such separations, evaporative dryers such as wiped film evaporative ("WFE") dryers have been employed in the past in order to try to isolate aromatic isocyanate monomers (such as toluene diisocyanate monomers) from polymeric toluene diisocyanate residue. Falling film evaporative dryers, such as thin film evaporative ("TFE") dryers, have been employed to try to isolate aliphatic isocyanate monomers, such as IPDI monomers, HDI monomers and H.sub.12 MDI monomers, from their respective polymeric residues. Unfortunately, these attempts have been less successful than might be desired since neither WFE nor TFE dryers enable the complete separation of isocyanate monomers from the polymeric residue. Indeed, the polymeric residue that is recovered from the WFE or TFE units typically contains approximately 30 to 50% by weight of monomeric isocyanate molecules based upon the weight of the residue. Thus, a significant amount of desirable monomer product typically remains unrecovered using these evaporative methods. Further, the unrecovered monomers cause residue to be in, or remain in, the form of a viscous paste that is difficult to remove from the evaporator. In order to remove this viscous paste from the WFE or TFE unit, the paste is typically solubilized in an organic solvent, such as chlorobenzene. Unfortunately, the residue containing such organic solvent is classified as toxic or hazardous waste, thus introducing a costly storage and/or disposal problem. As yet another alternative, the residue isolated from the WFE or TFE unit can be incinerated, but this alternative is costly and energy-intensive.
Another method of separating monomeric isocyanates from the solid polymeric residue is described in U.S. Pat. Nos. 4,871,828; 4,871,460; and 4,918,220 which disclose isolating isocyanate condensates using supercritical CO.sub.2 extraction. Unfortunately, this method is also energy intensive, and requires specialized equipment.
As another alternative, U.S. Pat. No. 4,774,357 discloses purification of crude polyisocyanates by extraction in a two-phase system. However, large volumes of organic extractant are required in such a process, posing the afore-mentioned problem of disposal of hazardous or toxic waste.
Heated mixer-kneader-type dryers are known to be useful in carrying out thermal processes on liquid, pasty, and solid materials. U.S. Pat. No. 3,687,422 discloses a multiple spindle mixing kneader that includes paddles, disc sectors, and kneading bars as product manipulation means. These elements are arranged so that the processed material is wiped on the heated inner surface of the housing to provide a large surface from which vaporization of volatile materials may take place. Other heated mixer-kneader dryers are disclosed in U.S. Pat. Nos. 3,317,959; 3,346,242; 3,347,528; 3,480,997; 3,689,035; 3,880,407; 4,575,253; and 4,650,338. U.S. Pat. No. 4,216,063 issued to Ailloud et al. discloses a method of continuously purifying toluene diisocyanate (TDI) to high purity using an agitated, scraped evaporator in combination with increasing evaporation temperature from within the range of about 70.degree. C. to 175.degree. C. up to a temperature within the range of about 210.degree. C. to 250.degree. C., using a pressure within the range of 1-50 mm Hg. The isolated residue product is said to be a dry, friable solid that is easily collected and disposed of. The process disclosed in the '063 patent, however, is limited to purification of one aromatic isocyanate, TDI, under the processing conditions described hereinabove. Attempts by the present inventors to adopt the process disclosed in the '063 patent for use on aliphatic isocyanates have been unsuccessful, resulting in a sticky, viscous paste that clogs the processing equipment, and is difficult to collect and dispose of.
In view of the above, it is clear that there is a need in the isocyanates manufacturing community for improved processes for isolating aliphatic isocyanate monomers from polymeric residue byproducts while simultaneously providing a waste residue that is easily handled and disposed of. The present invention provides one answer to that need.